This project explores the molecular mechanism oftwo ion-coupledtransporters: The(Na+ + K+)-coupledglutamatetransporter GLT-l from brainand theH+-organic cation multidrugantiporter EmrE from E. coli.These two systems provide unique experimentalparadigms forstructure/function relationships. In GLT-1wehavealready identified several amino acid residues lining the translocation pathway. Moreover, homologous bacterial transporters provide a powerful tool to applybacterial genetics to study the system. In thecase of EmrE, it is the smallest ion-coupledtransporter and displays unique stabilityand solubility properties. This makes it amenable to structural studies. We will progress towards understanding the molecular mechanism by (a) identification of residues in the translocation pathway, (b) in depth biochemical and electrophysiological analysis of the role of critical residues, (c) study of the helix packing and oligomeric structure and (d) high resolution structural studies. Regarding the recently obtained projection structure of EmrE at 7A resolution provides a promising starting point. In addition to impacting on the central question of the structural basis of ion-coupled transporter function, our studies may provide important clues for the role of these transporters not only under normal physiological conditions, but also in disease.